Depth of a potential repository with regard to the influence of subglacial overdeepenings and tunnel valleys in the representative preliminary safety assessments

The Site Selection Procedure for a repository for high-level radioactive waste in Germany consists of three phases. During Step 1 of Phase I, ninety sub-areas that cover 54% of Germany were identified. In the current Step 2 of Phase I, the ninety sub-areas will be reduced to a limited number of smaller areas. Within this step, so-called representative preliminary safety assessments (rvSU) are applied. During the rvSU, consecutive sets of criteria are applied in four evaluation steps to assess the extent to which the safe containment of radioactive waste can be expected in the areas for a period of one million years.

Safety assessments include the evaluation of potential impact of geological processes on the integrity of the geological barriers of a repository. A prominent process to consider is subglacial erosion, which leads to characteristic overdeepenings and glacial tunnel valleys. Such features were formed during the last glaciations during the Pleistocene in the North German Basin and the alpine Foreland. Subglacial overdeepenings can reach depths of over 500 m. They are formed by basal meltwater, which runs off under a glacier and can lead to deep channels being cut into the subsurface. During the erosion that leads to their formation, large quantities of sediment are transported away within short geological periods.

Therefore, they pose a potential threat to the integrity of the geological barriers of a repository, the overburden and the host rock.

The German Federal Institute for Geosciences and Natural Resources (BGR) has mapped the occurrence of subglacial overdeepenings in northern Germany (Breuer et al. 2023). On this basis, predictions were made about the extent and erosion potential of future overdeepenings. The result is a depth zone map, which shows the occurrence of overdeepenings in five zones, with the zone of the deepest expected structures having depths between 400 and 600 m below sea level. The continuation of the glacial cycles as observed in the Pleistocene is expected for the next one million years.

The occurrence of glacial overdeepenings is considered in several of the evaluation steps during the rvSU. The depth of influence of glacial overdeepenings can reach deeper than the legally prescribed minimum depth of 300 m, which is why the minimum required depth for areas with expected glacial overdeepenings is set deeper in the first evaluation step. The second evaluation step applies a further safety margin. In evaluation step 3, subglacial erosion is considered with the derivation of expected evolutions and the quantitative assessment. In evaluation step 4, the distance between the base of the overdeepening and the potential containment providing rock zone is considered, as well as the occurrence of erosion resistant strata, which should pose a hindrance of the formation to subglacial overdeepenings.

 

References:

 

Breuer, S.; Bebiolka, A.; Noack, V.; Lang, J. (2023): The past is the key to the future – considering Pleistocene subglacial erosion for the minimum depth of a radioactive waste repository. E&G Quaternary Science Journal, Bd. 72 (1). S. 113 – 125. ISSN 04247116. DOI: https://doi.org/10.5194/egqsj-72-113-2023